I get being irritated at a character for keeping secrets, but I don't feel like it's narratively artificial for the Ella doll to be doing so.
Even ignoring the fact that Ella is a manipulative cunt who loves fucking with people, I think she's probably concerned that MC is not in the right headspace to talk about his time with proto-SIN or whatever.
MC was probably in a bad place, having run away from home after his mom died, and I bet he did some morally dubious things with them. Just telling him "hey, once upon a time you were really into this SIN stuff, wanna join up again?" is very different from MC actually having experienced the emotions and memories that led him to want to join SIN in the first place.
She's probably concerned that without those memories, he'd just reject the idea completely. There's also the fact that the Ella doll still doesn't have any idea what Big Ella was really up to, for all she knows, she might not even want MC to remember anymore.